Conventionally, copyright protection of content has been aimed at, for example, by Content Scramble System (CSS) according to which content of a recording medium such as a digital versatile disk (DVD) is scrambled and rendered irreproducible so as to prevent an unauthorized copying thereof.
The conventional technique of copyright protection, though being capable of preventing unauthorized copying of the recorded content of the DVD, is unable to effectively block widely-practiced acts of piracy of the copyright of content in recent years. In recent years, infringers alter the structure of the DVD player itself that plays back the DVD contents so that the DVD player descrambles the DVD contents and produces plaintext contents. Then the infringers can copy the plaintext content on another recording medium such as a DVD and distribute the thus obtained pirate DVD content. Another widely observed act of piracy is an illegal peer-to-peer file exchange of content via the Internet. The conventional technique such as the CSS cannot effectively protect copyright of the content from such illegal acts. In addition, even when the pirate DVD is found, an altered DVD player employed for the manufacture of such pirate DVD cannot be identified from the content on the pirate DVD.
In order to eliminate such inconveniences, Advanced Access Content System (AACS), which is developed for a next-generation DVD video such as a High Density Digital Versatile Disk (HD-DVD), adopts a technique called Sequence Key as a technique to identify an altered DVD player which is employed for pirated production, based on Advanced Access Content System (AACS) Pre-recorded Video Book Revision 0.90 (Chapter 4).
The sequence key technique prepares plural contents that appear to be the same when observed though including minute difference with each other, and encrypts each of the plural contents with a different encryption key for recording on a recording medium. Each playback apparatus employed for playing back the content from such a recording medium, when adopting the Sequence Key technique, stores a key which allows for decryption of only one combination of contents among plural different contents. A plaintext content obtained via decryption by one playback apparatus is slightly different from another plaintext content provided via decryption by another playback apparatus. Thus, even when a pirate recording medium is manufactured via the copying of such content, a playback apparatus used for the decryption can be easily tracked back.